Radio frequency performance projection and stability trade-off of h-BN encapsulated graphene field-effect transistors

Hexagonal boron nitride (h-BN) encapsulation significantly improves carrier transport in graphene. This work investigates the benefit of implementing the encapsulation technique in graphene field-effect transistors (GFET) in terms of their radio frequency (RF) performance. For such a purpose, a drift-diffusion self-consistent simulator is prepared to get the GFET electrical characteristics. Both the mobility and saturation velocity information are obtained by means of an ensemble Monte Carlo simulator upon considering the relevant scattering mechanisms that affect carrier transport. RF figures of merit are simulated using an accurate small-signal model that includes non-reciprocal capacitances. Results reveal that the cutoff frequency could scale up to the physical limit given by the inverse of the transit time. Projected maximum oscillation frequencies, in the order of few THz, are expected to exceed the values demonstrated by InP and Si based RF transistors. The existing trade-off between power gain and stability and the role played by the gate resistance are also studied. High power gain and stability are feasible even if the device is operated far away from current saturation. Finally, the benefits of device unilateralization and the exploitation of the negative differential resistance region to get negative-resistance gain are discussed.Comment: 18 pages, 9 figure

Scaling of graphene field-effect transistors supported on hexagonal boron nitride: radio-frequency stability as a limiting factor

The quality of graphene in nanodevices has increased hugely thanks to the use of hexagonal boron nitride as a supporting layer. This paper studies to which extent hBN together with channel length scaling can be exploited in graphene field effect transistors (GFETs) to get a competitive radio frequency (RF) performance. Carrier mobility and saturation velocity were obtained from an ensemble Monte Carlo simulator that accounted for the relevant scattering mechanisms (intrinsic phonons, scattering with impurities and defects, etc.). This information is fed into a self consistent simulator, which solves the drift diffusion equation coupled with the two dimensional Poisson's equation to take full account of short channel effects. Simulated GFET characteristics were benchmarked against experimental data from our fabricated devices. Our simulations show that scalability is supposed to bring to RF performance an improvement that is, however, highly limited by instability. Despite the possibility of a lower performance, a careful choice of the bias point can avoid instability. Nevertheless, maximum oscillation frequencies are still achievable in the THz region for channel lengths of a few hundreds of nanometers.Comment: 29 pages, 7 figures, 1 table, Supplementary Information (10 pages) Funded by: 1 Micronova Nanofabrication Centre 2 European Union's Horizon 2020 (696656) 3 MINECO (TEC2013-42622-R, TEC2015-67462-C2-1-R, TEC2016-80839-P, MINECO/FEDER, FJCI-2014-19643) 4 MECD (CAS16/00043) 5 Generalitat de Catalunya (2014 SGR 384

Drain current saturation in graphene field-effect transistors at high fields

Development of competitive high frequency graphene field-effect transistors (GFETs) is hindered, first of all, by a zero-bandgap phenomenon in monolayer graphene, which prevents the drain current saturation and limits significantly the GFET power gain. An approach has been proposed to realise the drain current saturation in GFETs without a bandgap formation, but via velocity saturation of the charge carriers at high fields [1]. In this work, we report on the performance of GFETs fabricated using high quality CVD monolayer graphene and modified technology, which reduce the concentration oftraps generating the charge carriers at high fields [2]. Fig. 1 shows typical output characteristics of GFETs with gate length of 0.5 μm. The drain current clearly reveals the saturation trends at high fields, which we associate with the saturation of the carrier velocity, see inset to Fig. 2 [2]. Fig. 2 shows typical measured (extrinsic) transit frequency (fT) and the maximum frequency of oscillation (fmax), which are characteristics of the current and power gain, respectively. Since fT and fmax are proportional to the carrier velocity, they reveal similar saturation behaviour. We analyse the saturationeffects by applying the Fermi-Dirac carrier statistics. The fT and fmax are up to 34 GHz and 37 GHz, respectively, which are highest among those reported so far for the GFETs with similar gate length and comparable with those reported for Si MOSFETs [3]

El Síndrome del Burnout en Fisioterapia

El objetivo de este trabajo es identificar si existe el Síndrome Burnout en Fisioterapia y determinar qué variables personales y laborales inciden en su desarrollo. Para ello, además de los datos procedentes del cuestionario Maslach Burnout lnventory (Maslach y lackson, 19861, hemos recogido información relativa a diferentes caracterfsticas del puesto de trabajo y variables personales, todo ello en una muestra de 33 fisioterapeutas pertenecientes a centros públicos y privados de la CCAA de Murcia y provincia de Granada, durante el periodo Diciembre-Enero del año 2002. El perfil de la muestra es un sujeto varón, con una edad comprendida entre 24-34 años, casado, sin hijos, con un horario laboral de 5-7 horas, un número de 15-34 pacientes diarios aproximadamente, que trabaja en un solo centro, generalmente privado. En cuanto a los datos obtenidos del MBI podemos decir que el 42% de los profesionales de Fisioterapia encuestados presentan un nivel de cansancio emocional alto, el 63% muestran un nivel de despersonalización bajo y el 72% de los encuestados presentan un nivel de realización personal bajo. Al comparar nuestros resultados con los de otros autores, hemos detectado que las variables que inciden en el desarrollo de dichas actitudes son la edad, sexo, estado civil y el hecho de ser padres. Por tanto, existe un grado medio/alto del Síndrome Burnout en profesionales de Fisioterapia. Consideramos importante la formación con cursos de pre/postgrado.Terapia y Rehabilitació

Orbits in the hyperbolic plane

The authors characterize the orbits determined by some isometries in the hyperbolic plane as rotations with a fixed center, the limit rotations with a fixed point in the infinity line and the translations along a line. These orbits are called, respectively, circumference, horocicle and hipercicle. In addition, they show a exhaustive classiffication of the above isometries by means of the study of their fixed points. Some methods for building the above mentioned orbits are determined and some algorithms for their implementation are described. From these algorithms we have created some programming modules with the software Mathematica for the construction of such orbits and we have solved some constructive problems related with them

Does carrier velocity saturation help to enhance f(max) in graphene field-effect transistors?

| openaire: EC/H2020/785219/EU//GrapheneCore2 | openaire: EC/H2020/881603/EU//GrapheneCore3It has been argued that current saturation in graphene field-effect transistors (GFETs) is needed to get optimal maximum oscillation frequency (f(max)). This paper investigates whether velocity saturation can help to get better current saturation and if that correlates with enhancedf(max). We have fabricated 500 nm GFETs with high extrinsicf(max)(37 GHz), and later simulated with a drift-diffusion model augmented with the relevant factors that influence carrier velocity, namely: short-channel electrostatics, saturation velocity effect, graphene/dielectric interface traps, and self-heating effects. Crucially, the model provides microscopic details of channel parameters such as carrier concentration, drift and saturation velocities, allowing us to correlate the observed macroscopic behavior with the local magnitudes. When biasing the GFET so all carriers in the channel are of the same sign resulting in highly concentrated unipolar channel, we find that the larger the drain bias is, both closer the carrier velocity to its saturation value and the higher thef(max)are. However, the highestf(max)can be achieved at biases where there exists a depletion of carriers near source or drain. In such a situation, the highestf(max)is not found in the velocity saturation regime, but where carrier velocity is far below its saturated value and the contribution of the diffusion mechanism to the current is comparable to the drift mechanism. The position and magnitude of the highestf(max)depend on the carrier concentration and total velocity, which are interdependent and are also affected by the self-heating. Importantly, this effect was found to severely limit radio-frequency performance, reducing the highestf(max)from similar to 60 to similar to 40 GHz.Peer reviewe